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6 June 2012

The Transit of Venus

June 5th & 6th
2012 mark the last dates this century when the Earth, Venus, and the Sun will
be in perfect alignment. The next chance to view this celestial phenomenon will
not arise until the year 2117. And sadly, at the time of writing, the weather
forecast for the UK doesn’t look particularly favourable.

When the Sun rises today at 4:45 am
in London (4:30 am in Edinburgh, 5:15 am in Penzance) the transit will already
be in progress. If the clouds don’t spoil the show and we are able to observe
the sun, with the right apparatus and the correct filters or means of
projection (being very careful in order to avoid permanently damaging our eyes),
we should be able to see the passage of a tiny black dot passing serenely over
the face of the Sun.

The history of observing the
transit of Venus is equally as fascinating as understanding its significance in
the furthering of our scientific knowledge. Although many ancient cultures were
aware of, and so observed and studied the motion of the second planet of our solar
system, it seems none of them made any recorded observations of Venus
transiting the Sun (at least, none that we currently know of). The famous
German scientist, Johannes Kepler (1571-1630) was the first person to attempt
to predict the occurrence of the event. In 1621 he calculated that a transit
visible from the region of Europe would occur in 1631, but his calculations
weren’t accurate enough for anyone to actually observe the phenomenon. Through
refining Kepler’s predictions, an Englishman named Jeremiah Horrocks
(1618-1641) was able to make a more precise prediction, and he also established
that the transits occurred in pairs separated by eight years, recurring approximately
every 105 and 121 years.

The importance of accurately
observing such transits for the purposes of science were soon noted, and in
1668, the Scottish mathematician, James Gregory (1638-1675), suggested that
accurate observations of the transit of Mercury at different points on the
Earth’s surface could be used to better establish the distance of an
astronomical unit (that is, the distance between the Earth and the Sun). With
this knowledge in mind Edmond Halley (1656-1742), the English astronomer whose
name is most commonly associated with Halley’s Comet, set out to observe the 1676
transit of Mercury from the island of St Helena in the Atlantic; but he was
disappointed to find that only one other observation had been made, and
consequently he was not satisfied that this was enough data to make a
sufficiently accurate calculation. He proposed that the same system of
triangulation could be used in similar observations of Venus transiting the
sun, and, although he would not live long enough to make those
observations himself, there were others who took up his call.

In 1761, the Royal Society prompted
by the French astronomer, Joseph-Nicolas Delisle (1688-1768), along with a
number of other learned societies and scholars in different nations despatched expeditions
to various parts of the globe – in a truly notable early international
collaboration in the shared pursuit for scientific knowledge – to realise
Halley’s suggestion. There were more than 250 official observers placed around
the globe, many of whom had undergone great hardship and arduous travails
against the odds in their attempts to view the transit. We should take solace
therefore if our view is clouded out this year, as so were many of theirs; but
hopefully we shall only have to suffer the insult of the elements, whereas many
of these hapless astronomers-turned-adventurers also suffered the added
injuries of being caught in the crossfire of warring soldiers, or suffered attack from hostile
peasants, in all kinds of extreme climates!

It was during the transit of 1761,
however, that the existence of an atmosphere around Venus was first proposed. A
Russian scientist, Mikhail Vasilyevich Lomonsov (1711-1765) noted a curious occurrence
as the planet made its first and last contact (ingress / egress) with the solar
disc. He saw that an arc of light suddenly curved around the outer edge of the
planet, distinctly showing up against the dark background of space; the light
of the Sun having been harnessed and conducted by the planet’s atmosphere via a
similar principle used in today’s fibre optic technology. On the whole though,
the sum of the observations made in 1761 was simply a great disappointment.
None of the scholarly calculations made around the globe seemed to agree.
Undeterred though the scientific community resolved to try again in eight years
time.

And 1769 certainly saw a concerted
effort to redress the balance, particularly on the part of the Royal Society,
which organised thirty observation points in Britain alone, and more than
sixteen overseas. Again it was a truly international collaboration in which
many governments, overlooking their own national agendas and despite warring
differences, combined in funding a global effort to broaden mankind’s
scientific knowledge; a spirit which persists to this day, particularly in the
fields of astronomy and space exploration.

Perhaps the most famous of the
observing missions of 1769 was that undertaken by Captain James Cook
(1728-1779), who was engaged to sail HMS Endeavour to the southern hemisphere in company
with the botanists, Joseph Banks (1743-1820) and Dr Daniel Solander
(1733-1782). In June of that year Cook set up his observing station on the
island of Tahiti in the Pacific Ocean, recording the following entry for the
transit in his journal:

“Saturday
3rd. This day prov’d as favourable to our purpose as we could wish,
not a Clowd was to be seen the whole day and the Air was perfectly clear, so
that we had every advantage we could desire in Observing the whole of the
passage of the Planet Venus over the Suns disk: we very distinctly saw an
Atmosphere or dusky shade round the body of the Planet which very much
disturbed the times of the Contacts particularly the two internal ones. Dr
Solander observed as well as Mr Green and myself, and we differ’d from one
another in observing times of the Contacts much more than could be expected. Mr
Green’s telescope and mine were of the same Mag[n]ifying power but that of the
Dr was greater than ours.”

The difficulty in precision
regarding the timing of the contacts Cook describes (as shown in his drawings)
is actually due to the so-called ‘black-drop effect’ – an optical phenomenon
which you can replicate for yourself if you place your thumb and forefinger
close to your eye and then watch as you narrow the gap between them. If you
look carefully you’ll notice that the edges of your thumb and forefinger appear
to leap together before their surfaces actually meet. In reality this
phenomenon didn’t significantly alter the calculations made in 1769, and the
scientists of the day were able to determine the necessary trigonometry to
establish that the distance between the Earth and the Sun fell somewhere in the
region of 92,900,000 miles (149.5 million kilometres) and 96,900,000 miles
(155.9 million kilometres) – which isn’t far off from today’s estimate of
92,960,000 miles (149.6 million kilometres).

Despite the fact that nowadays we
have many different methods for studying our solar system and measuring its
distances, the actual observation of Venus transiting the Sun’s disc is still
important scientifically – not least as a marker of the efforts of those early
pioneers of science, but also as real science still. Such observations will aid
and inform astronomers who are currently engaged in the hunt for planets beyond
our solar system, many of which are now being discovered. It will also help
them to better determine the sizes of these ‘exo-planets’ more accurately. And
observations from Earth will compliment observations made by the Venus Express
(VEX) mission, an orbiter sent by the European Space Agency in 2005, which is
currently monitoring the Venusian atmosphere. And again such spectrographic
studies will assist in determining more about the atmospheres found around
exo-planets too.

In the time since Captain Cook
observed the transit of Venus in 1769 and the transits of our own era (2004
& 2012), exploration has enabled us to chart not only the Earth but also
our solar system and some of the stars beyond. We’ve mastered the powers of
flight and managed to send missions the surface of Venus itself, as well as
other planets. Watching that small black dot
transiting the solar disc or simply shining as the brightest of the planets
visible in our morning or evening sky, one can't help but wonder at the immense celestial mechanics
which regulate the passage of time and space within our solar system; and then wonder where our explorations will have
taken us by the time the next era of people look up to observe the passage of Venus across the Sun in 2117.

For more information about this
year’s transit (and that of 2004) take a look at the following page on NASA’s
website